1. Field of the Invention
This invention relates in general to method and apparatus for the contact-free potential measurement of an electronic component, preferably an integrated circuit, using an electron microscope wherein the energy of secondary electrons are measured with a spectrometer and in which a compensating circuit having a controlled-gain amplifier is arranged in circuit with the photomultiplier.
2. Description of the Prior Art
It is known that the potential on paths of an electronic component such as an integrated circuit can be measured with an electron microscope, particularly with a scanning electron microscope, in which the primary electron beam releases secondary electrons at the measuring location and the energy of the secondary electrons are determined by the potential at the measuring location. For energy measurements, a spectrometer, particularly an retarding field spectrometer, is utilized which contains a deflection capacitor which supplies the secondary electrons to an electron collector through an retarding field, a photomultiplier and a controlled gain amplifier which is connected to the electron collector. With the use of a compensating feedback loop circuit the output voltage of the controlled gain amplifier drives the retarding field of the spectrometer and maintains the grid voltage constant with reference to the voltage of the measuring location of the component. The grid voltage at the retarding field electrode of the spectrometer is adjusted until the voltage between the grid and measuring point reaches its original value. The change of the grid voltage then determines the potential change at the measuring location of the component which is the test object as described in the article "Scanning Electron Microscope" 1978, Volume I, SEM Inc., AMF O'hare Illinois 60666, USA, Pages 795 through 800.
In these known devices the operating voltage is applied to the component and the control clock pulses are supplied by a control clock unit circuit. By changing the photomultiplier voltage, a desirable operating point for the control loop which consists of the spectrometer and the photomultiplier as well as the preamplifier and controlled gain amplifier can be set according to the signal/potential characteristics of the control loop. The setting of this operating point is accomplished manually in the prior art devices.